The present invention relates to an improved tap for cutting internal screw threads.
Uncoated taps of normal hardness of the straight fluted and spiral point types have previously been used to cut internal screw threads. It has been suggested that the uncoated taps be made with increased hardness to increase the number of holes which could be threaded by a tap before the tap becomes excessively worn. Therefore, an uncoated tap normally having a Rockwell C surface hardness of 63 was hardened to 65 Rockwell C. The higher hardness tap was then used to tap holes. Although the harder surface of the tap better resisted wear, the tap was more brittle and tended to break easier under the cutting pressures applied. Therefore, it was concluded, for most purposes at least, that a conventional tap having a Rockwell C surface hardness of 63 was superior to the tap with higher hardness for the majority of applications.
In an effort to increase the number of holes which could be threaded with a tap, the conventional tap was given a black oxide coating. The characteristics of the black oxide coating, in certain environments at least, increased the number of holes which could be threaded by the tap.
In a further effort to increase the number of holes which could be threaded with a tap, Titanium nitride coating was applied to a conventional tap, that is a tap having a hardness of 63 Rockwell C. The Titanium nitride coating increased the number of holes which could be threaded with the tap. It is believed that the Titanium nitride coating increased the life due to the relatively hard surface provided by the Titanium nitride coating. In addition, it is believed that the Titanium nitride coating has an increased lubricity quality which decreases the torque required during the cutting of threads in certain materials.